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.Dd March 30, 2020
.Dt TRUNCATE 2
.Os
.Sh NAME
.Nm truncate ,
.Nm ftruncate
.Nd truncate or extend a file to a specified length
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn truncate "const char *path" "off_t length"
.Ft int
.Fn ftruncate "int fd" "off_t length"
.Sh DESCRIPTION
The
.Fn truncate
system call
causes the file named by
.Fa path
or referenced by
.Fa fd
to be truncated or extended to
.Fa length
bytes in size.
If the file
was larger than this size, the extra data
is lost.
If the file was smaller than this size,
it will be extended as if by writing bytes
with the value zero.
.Pp
The
.Fn ftruncate
system call causes the file or shared memory object backing the file descriptor
.Fa fd
to be truncated or extended to
.Fa length
bytes in size.
The file descriptor must be a valid file descriptor open for writing.
The file position pointer associated with the file descriptor
.Fa fd
will not be modified.
.Sh RETURN VALUES
.Rv -std
If the file to be modified is not a directory or
a regular file, the
.Fn truncate
call has no effect and returns the value 0.
.Sh ERRORS
The
.Fn truncate
system call
succeeds unless:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.It Bq Er EACCES
The named file is not writable by the user.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EPERM
The named file has its immutable or append-only flag set, see the
.Xr chflags 2
manual page for more information.
.It Bq Er EISDIR
The named file is a directory.
.It Bq Er EROFS
The named file resides on a read-only file system.
.It Bq Er ETXTBSY
The file is a pure procedure (shared text) file that is being executed.
.It Bq Er EFBIG
The
.Fa length
argument was greater than the maximum file size.
.It Bq Er EINVAL
The
.Fa length
argument was less than 0.
.It Bq Er EIO
An I/O error occurred updating the inode.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er EFAULT
The
.Fa path
argument
points outside the process's allocated address space.
.El
.Pp
The
.Fn ftruncate
system call
succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument
is not a valid descriptor.
.It Bq Er EINVAL
The
.Fa fd
argument
references a file descriptor that is not a regular file or shared memory object.
.It Bq Er EINVAL
The
.Fa fd
descriptor
is not open for writing.
.El
.Sh SEE ALSO
.Xr chflags 2 ,
.Xr open 2 ,
.Xr shm_open 2
.Sh HISTORY
The
.Fn truncate
and
.Fn ftruncate
system calls appeared in
.Bx 4.2 .
.Sh BUGS
These calls should be generalized to allow ranges
of bytes in a file to be discarded.
.Pp
Historically, the use of
.Fn truncate
or
.Fn ftruncate
to extend a file was not portable, but this behavior became required in
.St -p1003.1-2008 .
